The present invention relates to high-Tc superconducting ceramic oxide products, and macroscopic and microscopic methods for making such high-Tc superconducting products. The high-Tc superconducting ceramic oxide products of the present invention have a high critical current density, high critical magnetic field, long life and are capable of being recharged or having superconductivity regenerated.
Since the initial discovery of high-Tc superconductivity in metal-oxide ceramics, many people have tried to determine the underlying physical origin of this superconductivity. It is generally agreed that the microstructure of the CuO.sub.2 plane of high-Tc superconductors plays a key role in high-Tc superconductivity. Viewed in two dimensions, there are four oxygen atoms around a single Cu atom in high-Tc metal-oxide superconducting ceramics (in three dimensions, there would be six oxygen atoms around one Cu atom), and each Cu atom can supply at most three electrons to its nearest neighbors. This means that there can be no stable valence bond between the Cu atoms and the oxygen atoms. The Cu electrons are, therefore, only weakly localized and can pass across the oxygen bridges to complete quantum tunneling. Such collective quantum tunneling plays the key role in the high-Tc superconductivity. Since the exchange interaction between the two Cu ions is mediated via the oxygen ions, the extra spin of a hole localized on the oxygen will have a big effect. Designating the two Cu ions spins by S.sub.1 and S.sub.2 and the 0 by , the would prefer to be parallel or antiparallel in respect to both S.sub.1 and S.sub.2. The spins of high-Tc superconductors are, therefore, very disordered. The local spin wavefunction is either symmetric or antisymmetric and is rapidly changing with time, because of the mixed valence resonance vibration. The disordered spin wavefunction will be automatically adjusted to accompany the tunneling electrons.
The present invention relates to new methods of making completely sealed high-Tc superconducting products using metallic oxide ceramics, and to the completely sealed high-Tc superconducting products produced thereby. The inventive methods and products are based on the realization that the oxygen content of the metal-oxides plays an important role in high-Tc superconductors and products incorporating the same. Below a critical oxygen content X.sub.c1 (O), or above a critical oxygen content X.sub.c2 (O), superconductivity is destroyed. The transition temperature Tc changes in between these critical concentrations. For example, for the superconducting oxide system YBa.sub.2 Cu.sub.3 O.sub.x, X.sub.c1 (O)=6.5 and X.sub.c2 (O)=7.0. Experiments show that if oxygen atoms escape from high-Tc superconductors, thereby lowering the oxygen content to less than the critical oxygen content X.sub.c1, the superconductivity of the metal-oxide is destroyed. If the oxygen content is then increased, for example by sintering the oxygen-depleted metal-oxide ceramic within a predetermined temperature range in the presence of oxygen, the superconductivity will be restored. The principal point is that for YBa.sub.2 Cu.sub.3 O.sub.x superconductors, the oxygen content X(O) must satisfy the equation 6.5&lt;X(O)&lt;7.0, and for all high-Tc oxide superconductors the oxygen content X(O) must satisfy the equation X.sub.c1 &lt;X(O)&lt;X.sub.c2.
The high-Tc superconductivity state of oxide ceramics is only a metastable state, and the superconductive oxide ceramics will tend to lose oxygen to become a stable state insulator. This process of oxygen loss may take a few hours, a few days, a few months, or even a few years or longer depending upon the conditions surrounding the superconductive oxide including temperature, atmosphere, and the like. However, regardless of how long the oxygen loss process may take, the tendency of the metastable superconductive state to change to the stable insulative state is certain. Therefore, to protect the high-Tc superconductivity of oxide ceramics, the oxygen content of the ceramic corresponding to the superconductive state must be maintained.
The present invention provides a completely sealed superconducting product whereby the oxygen loss is prevented and a long-lived high-Tc superconducting ceramic oxide product is attained. As described in detail, hereinafter, the seal can be made using metal, plastic or any materials which are inert to oxygen.
The present invention is also based on the recognition that the high-Tc superconductors are ceramic materials, a basic property of which is brittleness. Because of this brittle characteristic of ceramic superconductors, many attempts were made to produce high-Tc superconducting ceramic products using traditional methods to make wires, cables, tapes and the like, and then making superconducting products from the superconducting ceramic-containing wires, cables and tapes. Examples of such wire, cable and tape methods of producing superconducting ceramic products include: U.S. Pat. Nos. 4,952,554; 4,965,249; 4,975,416; and 4,973,574. Other methods of making superconducting ceramic products are shown, for example, in the following U.S. Pat. Nos. 4,975,411; 4,975,412; 4,974,113; 4,970,483; 4,968,662; 4,957,901; 4,975,414; and 4,939,121.
However, all of these prior attempts to make high-Tc superconducting ceramic oxide products suffer from one or more disadvantages. The wire and cable making methods typically include a drawing or working step to reduce the diameter of the superconducting ceramic oxide product. Such drawing and working steps are liable to break the brittle ceramic oxide product, therefore the breaking and sintering cycles will repeat again and again and the resulting wires have poor flexibility and discontinuity caused by breaking. Further, prior attempts to produce superconducting ceramic products have not had the high mass density necessary to achieve high current density, have had an insufficient ratio of superconducting cross-sectional area to non-superconducting cross-sectional area, and have suffered undesirable oxygen loss resulting in loss of superconductivity. In addition, prior methods of making high-Tc superconducting ceramic oxide products have been costly, involving expensive materials and numerous, time consuming steps, and have produced products of only limited shapes suitable for only limited applications. Also, prior methods could not, or could not easily, make high-Tc superconducting connections, which is necessary, especially for making a high-Tc superconducting magnet. A key technology for making high-Tc superconductive magnets is the making of zero resistance connections.
This invention also attempts to apply an alternate or a selective waveform pulse magnetic field to destroy the magnetic moment order (which does not do good to the high-Tc superconductivity), to accelerate oxygen to occupy the positions of CuO.sub.2 planes, and to orient the CuO.sub.2 plane to a desired direction by the dynamic process of the alternate field during the heat treatment. This invention using a dynamic field has high efficiency compared with a static magnetic field. This is because dm/dt=.gamma.M.times.H(t), M is magnetization and H(t) is alternate field. The dynamics is very important; therefore, alternate field will rapidly rotate magnetic moment randomly, and create the condition to accelerate oxygen to occupy position on CuO.sub.2 plane, because AF local magnetic order resists diffusion of oxygen. Therefore, the applied alternate field is much better than an applied static magnetic field.